Frequency tunable semiconductor diode lasers provide versatile optical tools for telecommunications, metrology, spectroscopy and other uses. Many such tunable lasers use a diffraction grating with a movable reflector to select a desired wavelength from the beam diffracted by the grating. A diode gain medium is employed that has an antireflection (AR) coating on one facet thereof Light emitted from the AR coated facet is diffracted by a grating and directed to a movable reflector, which feeds light back to the grating and gain medium. Rotational movement of the reflector with respect to a pivot point selects the wavelength diffracted by the grating and allows the laser to be tuned to a desired output wavelength. Translational motion of the reflector is frequently employed in conjunction with the rotational motion to couple the cavity optical path length to the selected wavelength and provide mode-hop free tuning. Grating-tuned external cavity lasers are typically arranged in the Littman-Metcalf configuration with a xe2x80x9cfolded cavityxe2x80x9d, which permits compact-sized external cavity laser devices suitable for many commercial uses.
The optical output of grating-tuned external cavity lasers of this sort may be collected as the light emitted from a rear, partially reflective facet of the gain medium, or as the grating reflection of light directly from the gain medium. This provides a relatively high output power, but includes xe2x80x9cnoisexe2x80x9d in the form of source spontaneous emission (SSE) and amplified spontaneous emission (ASE) from the gain medium. One approach to providing a spectrally xe2x80x9ccleanxe2x80x9d output from grating-tuned external cavity lasers has been to simply insert a beam coupler directly into the laser cavity between the grating and gain medium. A partially reflective surface on the beam coupler directs a portion of the light returning from the grating outside the cavity. This partially reflected light is at the selected wavelength and has been spatially separated from the propagation direction of the spontaneous emission light by the grating. This spectrally clean output may then be coupled into a fiber for use in applications requiring high spectral purity.
This relatively simple approach to providing a spectrally pure output beam has an important drawback: the partially reflective surface of the beam coupler has the disadvantage of reciprocity. As a simple mirror, the beam coupler simultaneously reflects an equal portion of the beam traveling from the gain medium towards the grating. The insertion of a conventional beam coupler into the laser cavity thus always results in an optical loss from the opposite reflection off the partially reflective surface of the beam coupler from the spectrally cleaned light that is collected and use. The spectral cleansing provided by beam couplers thus is obtained with a corresponding sacrifice in laser output power.
There are many uses for external cavity lasers having output with high spectral purity, including medical, metrological and optical communications areas. The low power of currently available spectrally pure laser output has, however, limited the commercial use of external cavity lasers in these areas. There is accordingly a need for an external cavity laser apparatus that provides suppression of spontaneous emission light from laser output without significant optical loss, and which is simple, compact and inexpensive in design. The present invention satisfies these needs, as well as others, and overcomes the deficiencies found in the background art.
The invention is a laser apparatus and method that provides for suppression of source spontaneous emission (SSE) and amplified spontaneous emission (ASE) light in laser output with minimal intracavity loss. The apparatus comprises, in general terms, a gain medium emitting a light beam, a wavelength selection element positioned in the light beam, and a non-reciprocal pickoff positioned in the light beam to receive light returning from the wavelength selection element to the gain medium. The wavelength selection element may be tunable.
The non-reciprocal pickoff may comprise a linear polarizer positioned in the light beam, together with a non-reciprocal polarization rotator positioned in the light beam after the polarization-dependent beam splitter. The non-reciprocal pickoff may further comprise a reciprocal polarization rotator positioned in the light beam after the polarization-dependent beam splitter. The non-reciprocal polarization rotator and the reciprocal polarization rotator may be balanced with respect to each other, such that the non-reciprocal polarization rotator and the reciprocal polarization rotator each define substantially equal angles of polarization rotation. The rotators are configured to cancel out each other""s rotational effect on the polarization orientation of outward-bound light from the gain medium towards the wavelength selection element, and to produce an additive rotational effect on light returning towards the gain medium from the wavelength selection element. In certain embodiments, the gain medium and the polarization-dependent beam splitter may be angularly positioned with respect to each other at an angle that is equal or substantially equal to the angle of rotation defined by the non-reciprocal rotator, such that the gain medium and polarization-dependent beam splitter effectively provide the effect of a reciprocal polarization rotator.
The invention also provides methods of laser operation that comprise, in general terms, emitting a light beam from a gain medium along an optical path, positioning a wavelength selection element in the optical path, positioning a non-reciprocal pickoff in the optical path, feeding spectrally clean light back to the gain medium by the wavelength selection element, and picking off, by the non-reciprocal pickoff, a portion of spectrally clean light traveling the optical path towards the gain medium. The non-reciprocal pickoff may be positioned between the gain medium and the wavelength selection element. The positioning of the non-reciprocal pickoff may comprise positioning a polarization-dependent beam splitter in the optical path between the gain medium and the wavelength selection element, and positioning a non-reciprocal polarization rotator in the optical path between the polarization-dependent beam splitter and the wavelength selection element.
In certain embodiments the methods may comprise angularly positioning the polarization-dependent beam splitter and the gain medium with respect to the non-reciprocal polarization rotator at an angle that is substantially equal to the angle of polarization rotation defined by the non-reciprocal polarizer. The positioning of the non-reciprocal pickoff may, in other embodiments, comprise positioning a reciprocal polarization rotator in the optical path between the polarization-dependent beam splitter and the wavelength selection element. The methods may additionally comprise positioning a reflector in the optical path after the tuning element. In certain embodiments, the methods may further comprise defining an external laser cavity between the reflector and a reflective facet of the gain medium.
The invention also provides methods for generating spectrally clean laser output which, in general terms, comprise emitting a light beam from a gain medium outward along an optical path, allowing the outward traveling light beam to interact with a wavelength selection element, returning a spectrally cleaned light beam along the optical path to the gain medium from the wavelength selection element, and non-reciprocally picking off a portion of the returning, spectrally cleaned light beam from the optical path.
The non-reciprocally picking off may comprise passing the outward light beam through a linear polarizer such as a polarization-dependent beam splitter to linearly polarize the light beam, rotating the polarization orientation of spectrally clean light that is returned to the polarization-dependent beam splitter, and reflecting along an output path, by the polarization-dependent beam splitter, a portion of the returning light. The rotating may comprise passing the outward light beam and return light beam through a non-reciprocal polarization rotator. In other embodiments, the rotating may comprise passing the outward traveling light beam and return light beam through a non-reciprocal polarization rotator and a reciprocal polarization rotator.
Passing the outward light beam through the non-reciprocal polarization rotator and the reciprocal polarization rotator may comprise rotating, by the non-reciprocal polarization rotator, the polarization orientation of the outward light beam by an amount equal to +xcex8, and rotating, by the reciprocal polarization rotator, the polarization orientation of the outward light beam by an amount equal to xe2x88x92xcex8, such that zero net rotation is imparted to the outward light beam by the combined action of the non-reciprocal and reciprocal polarization rotators.
Passing the returning, spectrally cleaned light beam through the non-reciprocal polarization rotator and the reciprocal polarization rotator may comprise rotating, by the reciprocal polarization rotator, polarization orientation of the outward light beam by an angle +xcex8, and rotating, by the non-reciprocal polarization rotator, the polarization orientation of the outward light beam by an angle +xcex8, such that the combined effect of the non-reciprocal and reciprocal polarization rotators imparts a net rotation of +2xcex8 to the light returning from the wavelength selection element to the gain medium. Since the returning light beam has a polarization that has been angularly rotated with respect to the outward light beam, the returning light beam can be non-reciprocally picked off at the polarization-dependent beam splitter with minimal optical loss.
By way of example, and not of limitation, the gain medium may comprise a diode emitter or a flash-lamp pumpable or electrically pumpable crystal, dye, gas or other gain medium. The polarization-dependent beam splitter may comprise a thin film dielectric polarizer plate, a dichroic polarizer, a sheet polarizer, or other linear polarizer element. The non-reciprocal rotator may comprise a Faraday rotator or other magneto-optic or electro optic non-reciprocal rotator, or other device capable of non-reciprocal light polarization. The wavelength selection element may be tunable, and may comprise a grating, a thin film interference filter, a solid or gas etalon, an electro-optic element, a liquid crystal device, or other element capable of providing wavelength selection to light fed back to the gain medium. The reciprocal rotator may be balanced or substantially balanced with respect to the non-reciprocal rotator.
The invention may be embodied in a grating-tuned external cavity laser with an intracavity non-reciprocal pickoff in accordance with the invention. The external cavity laser apparatus may comprise, for example, a diode gain medium with a first, antireflection-coated facet emitting a light beam along an optical path, and a second, reflective or partially reflective facet. A linear polarizing beam splitter element is positioned in the optical path, with a non-reciprocal rotator element positioned in the optical path after the linear polarizing element, and with a reciprocal rotator element positioned in the optical after the non-reciprocal rotator element. A grating is positioned in the optical path after the reciprocal rotator, followed by an end tuning reflector or mirror. The end reflector and the reflective facet of the gain medium define an external laser cavity. The order of the non-reciprocal rotator and the reciprocal rotator elements positioned between the polarizing beam splitter and the grating element may be reversed, with the non-reciprocal rotator positioned after the reciprocal rotator.
In operation, the gain medium outputs light that includes source spontaneous emission (SSE) and amplified spontaneous emission (ASE) associated with current pumping of the gain medium. Light returned or fed back along the optical path to the gain medium from the grating element is spectrally pure at the wavelength selected by the tunable element, with the SSE or ASE light components having been substantially removed by the grating. The polarization-dependent beam splitter passes linearly polarized light from the gain medium (which contains SSE and/or ASE components) along the optical path to the non-reciprocal and reciprocal rotators. The polarization dependent beam splitter does not reflect or substantially reflect the outward-bound light from the gain medium from the optical path, which allows for a greater amount of spectrally clean light to ultimately be returned back towards the gain medium.
The non-reciprocal and reciprocal rotators are configured to provide a xe2x80x9cbalancedxe2x80x9d rotator pair to the light traveling outward from the gain medium and polarizing beam splitter towards the grating or other wavelength selection element, such that each rotator provides an equal, but opposite, amount of polarization rotation to the light beam. The net effect of the non-reciprocal and reciprocal rotator pair on the linearly polarized light traveling towards the grating is zero rotation, i.e., the polarization orientations introduced by the rotators effectively cancel each other out. In this manner, the light traveling towards the grating is optimally polarized for effective diffraction by the grating.
As spectrally clean light is returned along the optical path from the grating towards the gain medium, the non-reciprocal and reciprocal rotators act additively and impart in a net change in polarization orientation to the light beam. The polarization-dependent beam splitter passes only a portion of this returning light of changed polarization orientation, and the remainder of the returning light is non-reciprocally reflected or picked off by the polarizing beam splitter as spectrally pure optical output. This spectrally pure output is directed down an output path and may be coupled into a fiberoptic. The proportion of light that is non-reciprocally picked off rather than returned to the gain medium may be controlled by selection of rotation angle provided by the rotator pair.
In other embodiments wherein a grating is used for a wavelength selection element, the polarizing beam splitter and gain medium may be rotationally oriented with respect to the non-reciprocal rotator by an amount equal to the rotation of polarization orientation provided by the non-reciprocal rotator. In this configuration, the gain medium and polarizing beam splitter provide the effect of a reciprocal rotator and eliminate the need for a separate reciprocal rotator. Alternatively, the rotational orientation of the grating with respect to the non-reciprocal rotator may be varied to accommodate the polarization rotation introduced by the non-reciprocal rotator.
In embodiments of the invention that use wavelength selection elements which are not polarization dependent, such as interference filters and etalon tuners, the reciprocal rotator may be omitted, as the polarization orientation of the outbound light beam does not effect tuning efficiency. Thus, the apparatus of the invention may be embodied in an external cavity laser apparatus comprising a diode gain medium emitting a light beam along an optical path, a polarizing beam splitter element positioned in the optical path, a non-reciprocal polarization rotator positioned in the optical path after the polarizing beam splitter, a tunable filter positioned in the optical path after the non-reciprocal rotator, and an end reflector positioned in the optical path after the tunable filter. In operation, the light beam traveling outward from the gain medium is linearly polarized by the polarizing beam splitter, and then has its polarization orientation rotated by the non-reciprocal beam splitter. The outward beam then passes through the tunable filter, reflects off the end mirror and returns through the tunable filter. The spectrally clean return beam is then further rotated in its polarization orientation by the non-reciprocal rotator, after which the polarizing beam splitter picks off a portion of the spectrally clean return beam and directs this portion down an output path. The remainder of the return beam is fed back to the gain medium.
The use of a non-reciprocal pickoff as provided by the invention provides a spectrally pure laser output of relatively high power with minimal intracavity loss. The non-reciprocal pickoff of the invention utilizes simple, inexpensive optical components that have large alignment tolerances, and which are suitable for very broad-band applications. Other objects and advantages of the invention will be apparent from the detailed description below.